I ran into this common belief today, and was surprised that it has not been addressed here before.

I think this belief is common enough that most anyone has heard it. Especially if they are touring a medieval villa in Europe, or other ancient structure with glass windows. Invariably someone will state that the panes are thicker on the bottom than the top because the glass is slowly flowing.

This is even an idea spread in Ed Greenwood's description of the "Elves of Myth Drannor" where he imagines an art form where the long lived elves would leave glass out for centuries to flow and attain specific shapes (sorry, I sold my print version of that Forgotten Realms source-book and can't find it on-line).

There's a difference between liquid and fluid. Glass is meant to be the latter. In any case I've restricted the question and answer to the on-topic, non-definition part - fluids include amorphous solids, like glass.
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Sklivvz♦Jan 12 '12 at 7:48

@Brightblades - for some reason we have very few if any castles in the US built in medieval times. If only we had an oppressive tyrant to create a caste system forcing the ill bred into a feudal system, and structures of defense that also served to reinforce the idea of unchallengeable power.
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ChadJan 12 '12 at 16:33

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@Brightblades I am fairly sure of my statement. If a castle nowadays has glass on the windows, it's either not a castle (a villa or a church) or the glasses are modern. Most medieval castles did not have windows has they became common only late in the game (XIV century). In any case, if you go to visit a castle today and you see a glass window it's quite likely that it's modern.
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Sklivvz♦Jan 12 '12 at 19:56

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@Sklivvz we may be talking past each other. :-) To us uncouth Americans, some of those giant residential villas are also castles. Not only the military fortifications. Maybe that's the source of my confusion. (And yes, I realize that uncouth American is redundant!)
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BrightbladesJan 12 '12 at 20:31

Molten glass is gathered on a blowpipe, and blown to an elongated balloon shape . The ends are cut off and the resulting cylinder is split with shears while still hot, then flattened on an iron plate. This is the forerunner of the Cylinder process. The quality of the glass was not good, with many imperfections.

(By the way, finding original source documents on actual manufacturing of glass is bloody difficult... Many reference pages are dead, and most other pages are referring to this particular "flow" question.)

There is no clear answer to the question “Is glass solid or liquid?”. In terms of molecular dynamics and thermodynamics it is possible to justify various different views that it is a highly viscous liquid, an amorphous solid, or simply that glass is another state of matter that is neither liquid nor solid. The difference is semantic. In terms of its material properties we can do little better. There is no clear definition of the distinction between solids and highly viscous liquids. All such phases or states of matter are idealisations of real material properties. Nevertheless, from a more common sense point of view, glass should be considered a solid since it is rigid according to everyday experience. The use of the term “supercooled liquid” to describe glass still persists, but is considered by many to be an unfortunate misnomer that should be avoided. In any case, claims that glass panes in old windows have deformed due to glass flow have never been substantiated. Examples of Roman glassware and calculations based on measurements of glass visco-properties indicate that these claims cannot be true. The observed features are more easily explained as a result of the imperfect methods used to make glass window panes before the float glass process was invented.

“The idea that glass is a fluid is a very widespread myth,” says Yvonne Stokes, a mathematician and spoilsport at the University of Adelaide in Australia. “I was told it as a fact by my adviser. And once, a class of schoolchildren came into the lab, and one of them told me the very same thing. If you want to talk microscopically, then you can call glass a fluid. But people understandably tend to think that if it’s a fluid, it flows. It’s that notion that’s false.”Stokes has recently proved with detailed calculations that old windows could not have flowed perceptibly.

If the myth survives, it will be because it contains a kernel of truth – and because glass is a confusing kind of matter, quite unlike the three ordinary kinds. A gas is an anarchy of molecules going every which way; a liquid is a tighter but still disorderly society in which molecules constantly dissolve and reestablish weak bonds; a solid is a molecular army in rigid formation.But glass is … none of the above. It is rigid like a solid, but its molecules are not arranged in repeating crystals. It is amorphous like a liquid.

In fact, structurally there is no sharp line between a liquid and a glass. You form glass by ‘super cooling” a liquid below its freezing point, then cooling it some more. If you cool it fast enough, the molecules can’t organize themselves into crystals. As the temperature drops, the liquid becomes more viscous and the molecules more sluggish. It’s like a game of molecular musical chairs in which the music never stops and the players never sit down; instead they seem to move through honey, then tar, until they are all but motionless, like bugs in amber.

Old glass window-panes are thicker at the bottom because the techniques to create glass with an even thickness are a rather recent innovation.

Basically, Prior to the invention of the float-glass process, glass was made by either pouring molten glass into a flat-bottomed mold, running molten glass between rollers, or spun to produce a disc shape through centripetal force.
This produced a sheet of glass with varying thickness.
Then, when the glazier went to cut and mount the glass into window panes, the thicker end of each pane was almost invariably mounted at the bottom.

There are occasional instances of antique windows being found with the thicker end of the pane mounted at the top.

The observation that old windows are sometimes found to be thicker at the bottom than at the top is often offered as supporting evidence for the view that glass flows over a timescale of centuries. The assumption being that the glass was once uniform, but has flowed to its new shape, which is a property of liquid.[40] However, this assumption is incorrect; once solidified, glass does not flow anymore. The reason for the observation is that in the past, when panes of glass were commonly made by glassblowers, the technique used was to spin molten glass so as to create a round, mostly flat and even plate (the crown glass process, described above). This plate was then cut to fit a window. The pieces were not, however, absolutely flat; the edges of the disk became thinner as the glass spun. When installed in a window frame, the glass would be placed thicker side down both for the sake of stability and to prevent water accumulating in the lead cames at the bottom of the window.[41] Occasionally such glass has been found thinner side down or thicker on either side of the window's edge, the result of carelessness during installation.[42]

Mass production of glass window panes in the early twentieth century caused a similar effect. In glass factories, molten glass was poured onto a large cooling table and allowed to spread. The resulting glass is thicker at the location of the pour, located at the center of the large sheet. These sheets were cut into smaller window panes with nonuniform thickness, typically with the location of the pour centred in one of the panes (known as "bull's-eyes") for decorative effect. Modern glass intended for windows is produced as float glass and is very uniform in thickness.

Your answer is probably mostly correct but you need references to support your claims about earlier processes producing uneven glass and the "Invariably mounted at the bottom" with the variation of it being mounted at the top some time. Wikipedia is good support but not for primary reference.
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ChadJan 12 '12 at 16:26